The present invention relates to an exhaust manifold for internal combustion engines. More specifically, the present invention relates to an exhaust manifold for internal combustion engines that allows early activation of a catalyst immediately after starting, reduces weight and costs, increases joining precision between plate materials, and improves welding precision.
Internal combustion engines have an exhaust manifold to collect exhaust gas discharged from gas columns. A catalyst is disposed following the exhaust manifold to purge harmful components of the exhaust gas from the exhaust manifold.
Exhaust manifolds for internal combustion engines include exhaust manifolds integrally formed by casting, as well as exhaust manifolds formed by joining a plurality of plate materials.
Examples of these exhaust manifolds for internal combustion engines are disclosed in Japanese examined utility model publication number 8-7055, Japanese laid-open patent publication number 10-89064, Japanese laid-open patent publication number 8-260958, Japanese laid-open patent publication number 9-317462, and Japanese laid-open patent publication number 10-89060.
Japanese examined utility model publication number 8-7055 discloses two plate materials joined to form an exhaust pipe collecting section. Of the two plate materials, the one that faces the main engine unit is formed thinner than the plate material positioned on the other side of the main engine unit. This difference in thickness generates a difference in vibration frequencies between the two plate materials, thereby reducing vibration noise. Also, the thicker plate material on the opposite side from the main engine unit restricts the transmission of exhaust noise.
Japanese laid-open patent publication number 10-89064 discloses the joining together of a front half, a partitioning body, and a rear half, each formed as plates. A plurality of exhaust pipes and confluence sections between two exhaust pipes are formed from the partitioning body and either the front half or the rear half. This allows for the reduced thickness and weight of the exhaust manifold.
Japanese laid-open patent publication number 8-260958 discloses junctures between the branching pipes that are made thicker than the other sections. The thickness is made greatest at the juncture disposed at the longitudinal center of the cylinder head. This increases the compressive stress generated at the junctures.
Japanese laid-open patent publication number 9-317462 discloses an outer pipe and an inner pipe, supported in the outer pipe, so that the two are separated by a gap. The outer pipe is formed so that the gap is larger near the engine attachment flange. This allows the thermal transfer from the inner pipe to the outer pipe to be reduced while allowing the exhaust temperature guided to the catalyst to quickly rise when the engine is started.
Japanese laid-open patent publication number 10-89060 discloses a vertically oriented engine having an exhaust port with an exit-side opening positioned higher toward the front or the rear of the automobile. The branching pipes, continuous with the exit-side opening, are positioned outward. This reduces the overlap between the branching pipes, when seen from the front of the automobile. As a result, the variations in the running airstreams that come into contact with the branching pipes are reduced and thermal warping is prevented.
Internal combustion engines use a catalyst to reduce harmful elements in the exhaust gas. The catalyst efficiently purges harmful elements when the catalyst temperature reaches its activation temperature.
In recent years, there has been an increasing demand for reducing harmful elements in the exhaust gas. In particular, there has been a demand to reduce the harmful elements in the exhaust gas that is discharged immediately after an internal combustion engine is started, since, at this time, the catalyst temperature is too low for the catalyst to be effective in removing the harmful elements.
For this reason, an exhaust manifold for internal combustion engines is formed by joining plate material having a smaller heat capacity than that of an exhaust manifold formed by casting. This allows the catalyst temperature to rise to the activation temperature quickly after the engine is started, thus providing early activation of the purging effect.
Referring to FIG. 12, there is shown an example of this type of exhaust manifold for internal combustion engines. Referring to FIG. 12, there is shown an engine compartment 102 for an automobile (not shown in the figure). An internal combustion engine 104, a cylinder head 106, and an exhaust manifold 108 are mounted in engine compartment 102. Exhaust manifold 108 is formed by welding an upper case 110 and a lower case 112. Upper case 110 and lower case 112 are formed as two metal sheets.
Exhaust manifold 108 attaches to cylinder head 106 with a head attachment flange 114. A catalyst (not shown in the figure) is attached to a catalyst attachment flange 116. When internal combustion engine 104 is mounted sideways in engine compartment 102 toward the front of the automobile (not shown in the figure), exhaust manifold 108 is disposed to the front of internal combustion engine 104.
Exhaust manifold 108 is generally formed so that a sheet thickness t1 of upper case 110 and a sheet thickness t2 of lower case 112 are identical (t1=t2). Exhaust manifold 108 is cooled by air currents flowing through engine compartment 110 such as cooling air from a radiator fan (not shown in the figure) and running airflow.
Since upper case 110 is positioned further toward the front than lower case 112, relative to the direction of the air currents, upper case 110 is cooled more than lower case 112.
The stress tolerance of the metal sheets forming upper case 110 and lower case 112 increases for lower temperatures. Also, the heat capacity of the metal sheets is smaller if the thickness of the sheets is smaller.
Upper case 110 and lower case 112 are formed with the same sheet thickness (t1=t2) based on the stress tolerance of lower case 112, which receives less cooling from air flows. As a result, there is excess strength in upper case 110, which is cooled more than lower case 112, and therefore has a larger stress tolerance. This increases the amount of required materials, the weight, and the production costs. Furthermore, upper case 110 has a higher heat capacity. In particular, the temperature of the exhaust gas sent to the catalyst immediately after internal combustion engine 102 is started is reduced, thus lengthening the time required for the catalyst to be heated to its activation temperature.